Color photography



Patented Mar. 2, 1943 cocoa rno'rocaarnr Albert L. Bunting, Detroit,Mich, assignor to Union Research Corporation, Detroit, Mich., acorporation of Michigan No Drawing. Application April 27, 1940, 1 SerialNo. 332,067

8 Claims.

The present invention relates to new and useful improvements in colorphotography, relating more particularly to a photographic process inwhich an initial photographic image is reproduced in the terms of thecolor blue, as well as relating to a colored photographic printresulting from the process.

It is well known in the art of color photography that a. plurality ofcolored images each of which is reproduced in a primary color, such asblue, yellow or red, may be assembled, superimposed in registry, orotherwise combined to produce a completed colored'image which covers thefull color range of the visible spectrum. These initial colored imagesmust be carefully selected as to .color value and intensity so that whencombined a balanced colored print is produced.

The blue photographic print of the present invention may be used in anydesired method of assembly or combining of the separate color imagesutilized to produce such a finished colored print. The other initialcolored images also used for this purpose, may be produced by any knownprocess which provides colors suitable for use with the blue colorprints herein disclosed, and may also be produced according to thedisclosures of my co-pending application Serial No. 332,069 andaccording to the disclosures of thecopending applications of myself andRaymond W. Thomas, Serial Nos. 332,068, 332,071 and 332,070.

It is one object of the present invention to agent and a solvent for thecompounds which form no part of the blue colored compounds of the blueimage thereby preventing such compounds from hampering or preventing theformationof the blue image; and to providesuch a process in which a.bleached initial black-and-white image may be. oxidized prior tosubjecting it to the combined action of the blue color producing agentand solvent.

A still further object of the invention is to provide a bluephotographic print of the nature provide an improved blue photographicprint and a simple, effective and convenient method for the productionthereof, the method being readily controlled to produce a blue color ofpredetermined color values and light transmitting properties.

It is another object of theinvention to provide a process for producinga blue photographic print which includes a conversion of an initialconventional photographic black-and-white image to a blue image whichfaithfully reproduces the graduations and tones of the initial image andwhich is light-fast and capable of repeated satis factory exhibitions oruses particularly photo-.

graphic projections.

Another object of the invention is to provide a process for producing ablue photographic print n which a bleached initial black-and-white imagebythe combined action of a blue color producing 55 referred toabovewhich is relatively inexpensive to manufacture and which is adaptablefor use in the production of all kinds of color photographs such,'forexample, as paper prints and transparencies.

Other objects and advantages of the invention will become apparentduring the course of the a following description and of the appendedclaims forming a part hereof.

GENERAL DISCUSSION or PRESENT Pnocrss cyanide radical. This intermediateimage is reactedlwith ferrous or ferric ions to produce a substantiallywater insoluble blue colored image believed to be ferriferrocyanide,ferroferricyanide,

- or'mixtures thereof as explained in detail hereinafter. The initialimage may consist of any compound capable of being. reacted with areagent suitable to form a substantially water insoluble complex ironcyanide.

Plionucrron or rim INITIAL IMAGE One satisfactory method of producingsuch an initial image is to produce a conventional developed,black-and-white image. This image is preferably a developed silverimage. In carrying out the present invention, the initial image isconverted to an image consisting chiefly of a 'substantially waterinsoluble ferrocyanide or ferricyanide. This conversion is oftenreferred to in the art as bleaching. Among other bleaching. agents whichmay be employed are chromic acid, chromates and halide acids. However,images. bleached by agents other than those having acomplex iron cyanideas a part of their composition should be treated to form a complex ironcyanide in a separate step. Whether this complex iron cyanide be formeddirectly from-the developed black-and-white image or by suitablytreating a bleached image, it is then treated with a solution containingferrous or ferric ions for the production of a blue colored image.

Because a blue positive image made in accordance with the presentinvention is a faithful reproduction of the initial black-and-whiteimage, it will be understood that the intensity or density of theinitial image determines the density or intensity in the blue onederived therefrom. Therefore, the control of the density of the initialimage by anyconventional method suitable for such a purpose controls thecolor intensityof the image in the blue positive print. Intensitycontrol ofthe initial image may be accomplished by conventionalphotographic procedure as, for example. by control of exposure times.mode of development, choice of emulsions, or by combinations of thesefactors and in other ways well known in the art.

In preparing the initial black-and-white posi-. tive image any suitablefilm or plate may'be used. Motion picture positive stock which possessesbut a single coating of emulsion has been used successfully and isrecommended for this purpose, particularly the film known as Eastmanpositive nitrate film No. K1301. Also, reversible film stock givessatisfactory results. The lightsensitive material contained in theemulsions of these films is preferably a silver compound or mixturesthereof although any suitable lightsensitive material may be utilized.

- Drzvstorusn'r or Im'rnr. hues For purposes of developing the initialimage referred to. any standard or conventional developing solution maybe employed, but I have found that the developers known as D-ll andDK-20 of the Eastman Kodak Company produced, particularly good results.However, the so-called M-Q developer is also satisfactory. Thisdevelopment may be accomplished in trays.

image, according to the present invention the initial image is convertedinto a water insoluble complex iron cyanide image by immersion in asolution containing iron cyanide ions. Assuming that the initial imageis a developed silver image and the bath consisted of an aqueoussolution of potassium ferricyanide the converted image would be silverferrocyanide. The strength of the converting solution is not critical,as the reaction goes to completion. Although I do not fully understandthe exact nature of this con- This equation is given as typical of thegeneral reaction which other complex iron cyanides undergo with silverand is intended to be illustrative in nature and not limiting. Moreover,other converting agents, some of which have been mentioned hereinbefore,may be used, provided the image is then treated with a solution suitedto produce "a complex iron cyanide image of the in a tank, or by anysuitable procedure. In the of fourteen minutes was employed also, atabout 18 C. The above times app y to the D-ll developer. However.suitable development times for any given developer employed as .well assuitable technique and procedure therefor may be readily worked out byone skilled in the art. Also a developer may be selecetd to produce adesired efiect such as a fine grain, desired intensity and contrastvalues and the like, since thecharacteristics of developers are known inthe art.

After development, the initial image is washed, placed in a conventionalshort stop bath, such as a weak aqueous solution of acetic acid, and isagain washed, according to standard photographic practice.

If colored images other than a blue one produced in accordance with thepresent invention are to be produced in the emulsion layer containingthisblue image. the film should not be fixed at this stage of myprocess. Otherwise. the film may be fixed at this stage by immersion ina suitable hypo bath. Fixing at this point is recommended if it can bedone, since the rebetter blue color print results.

Common or me 1mm. Inca general type specified above.

It'is desirable that the iron cyanide'hleaching bath be buifered toproduce a basic solution because the iron cyanide solution is morestable if. basic. If it be neutral or acid it tends to break down, forinstance by reacting with the carbon dioxide of the atmosphere. Also.the iron cyanide bath has been found to bleach the initialblack-and-white image better if basic. In this bleaching reaction themetal forming the blackand-white image is oxidized to form a complexmetal iron cyanide. Therefore the bleaching reagent is itself reducedand is an oxidizing agent. An iron cyanide, used herein as the bleachingagent, is a better oxidizing agent if in a basic solution. Although anybufiering agent suitable to produce a basic solution may be used, alkalicarbonates, bicarbonates or hydroxides are recommended.

The complex iron cyanide image produced by the above step isgrayish-white in color but is otherwise a faithful reproduction of theinitial silver image. Substantially all of the silver of the initialimage is converted to silver iron cyanide complexes and the print isleft in the solution until the reaction goes to completion.

The converted film is next washed in order to remove all surplusferricyanide, or a surplus of other converting materials or solublereaction products which may remain within or upon the film. However, acomplete removal thereof by an extended washing is not essential at thispoint provided that any excess ferricyanide or other soluble complexcompound or excess reagent which may remain isremoved prior to the bluecolor producing bath to prevent staining.

' The image is next further converted by treatment with an oxidizingagent, preferably an aqueous solution of nitric acid or chromic acid.The concentration of this solution is important only at the upper limitwhich should not exceed approximately 50% (by volume) of pure nitricThis reaction goes to completion and the time For thepreparaticn of theblue-and-white to of the immersion therefore is not critical providedthat the film is not-left in the solution so long that the gelatin,emulsion, or carrier is attacked or stripped. I believe this oxidationreaction produces a different complex cyanide radical from that of thebleached image, probably a radical in which the valence of one of theelements has been increased. Moreover, the dual-character of the formercomplex AgzK [Fe (CN) B] the oxidized bleached image is allowed to standthe reaction which causes this change in color of the image.

AgK[Fe(CN)e] range The initial metallic black-and-white image havingbeen bleached with ferricyanide or bleached with some other suitablebleaching agent and treated with a ferricyanide to form an imageconsisting of a complex metal cyanide,

may be oxidized to producethe oxidized orange image by subjecting thelatter to the action of any oxidizing agent capable of oxidizing thecomplex cyanide; For instance, a bleached image consisting of silverferrocyanide may be oxidized to an image consisting of silverferricyanide. In.

that one skilled in the art may select one ,of

these or other oxidizing agents suitable for oxidizing the gray-whitebleached image to the oxidized orange image.

The print is next given a wash to remove the excess of nitric acid orother oxidizing agent as well as any soluble reaction products from thebleaching reaction which may remain in or on the film. In order todetermine whether or not the film is sufficiently washed at this point,

drippings from the washed film may be added to 'a solution containingferrous ions. If the solution turns blue or bluish the wash isincomplete. After this wash the film is placed in an acidified aqueoussolution containing ions of ferrous-or ferric iron, the concentration ofwhich is not critical, although the rate of converting the orange-silverferricyanide image into the desired blue image is increased if the ionicconcentration of the ferrous or ferric iron is relatively high. Thisreaction is permitted to go to completion which may be determined by theappearance of the blue colored image. Thereafter, the blue image iswashed to remove the soluble iron salts and is placed in a conventionalhypo bath until it becomes clear and transparent and thereafter iswashed and dried; The resulting print is a transparent print in whichthe image appears in a blue scale exactly corresponding to the grayscale of th initial black-and white print.

It is recommended that ferrous ions be used. however. If. oxidation ofthe bleachedimase is complete at the oxidizing operation, ferric ionstend to produce an inferior image. I have found, however, that ifoxidation is not complete, or if too long before being immersed in theblue produclng bath, that ferric ions may be employed fro-provide a fairblue image. Apparently, the oxidized bleached image tends to reduce to acertain extent on standing. Complete oxidation and the use of ferrousions is preferable inasmuch as more constant and more easily controlledblue color values are produced by this procedure. Ihave found that theintensity of a blue image produced by using ferrous ions may beintensified somewhat by immersing the film, after the production of theblue image,- in an acid bath or by utilizing a long wash after the hypooperation. It is also pointed out that a conventional hypo solutiontends to-reduce the blue color forming compound which does not destroythe blue color, but does make control of color values more diillcult.Therefore, the use of an acid hypo bath is recommended.

EXPLANATION or BL E COLOR The reaction producing the blue precipitatemaking up the blue image is represented, so far as now. known, by eitherof the following equations, depending upon the condition of thereaction.

Aga[Fe (CN)5]+FeS0 b FB:4[F6(CN)012 (Turnbuil's Blue") Aga[Fe(CN)]+Fe:(S0i): -o Fe4[Fe(CN)s]z ("Prussian Blue") Thus my blue colorproducing compound is believedto be a mixture of Prussian blue andTumbull's blue as it can be produced by either ferrous or ferric ions.Inasmuch as ferric ions plus a ferricyanide gives a brown color, areduction to ferriferrocyanide (Prussian blue) appears to take placewhere ferric ions are used. However, if oxidation of the bleached imageis complete and ferrous ions are employed in the blue forming bath, Ibelieve that the blue color roducing compound is substantiallycompletely Turnbulls blue. While the concentration of the ferrous orferric ions in the blue color forming 1 bath mentioned affects the rateof the reaction, that concentration does not appear to affect thecharacter of the resultant .blue image. This concentratlon may be variedwithin relatively wide limits without materially ailecting the saidconversion.

It should be pointed, out, however, that the following reaction inaddition to those mentioned probably occurs:

(Potassium ferriierrocyanide) The compound produced by this reaction isalso blue and my blue color producing compound probably contains arelatively small amount of it. This reaction appears to occur sincepotassium ferrocyanide is capable of oxidation.

The present process, as will be readily understood'after a considerationoi the foregoing disclosure, particularly .utilizes the reaction of anyferrous or ferric ion with a complex iron cyanide. Therefore, anysuitable iron salt producing ferrolls 'or ferric ions dissolving, plusany suitable image having a complex iron cyanide, prefer'ably aife'rrior i'errocyanide, as a'radical inits image producing compound maybeemployed.

For instance, an image composed of the reaction instance.

ALTERNATE PROCESS FOR PRODUCING THE BLUE IMAGE Another satisfactorymethod of carrying out the present invention is as follows: An initialblack-and-white" positive print is made in the way explained above. Thisimage is treated with potassium ferricyanide or some other bleachingagent as above disclosed and is preferably oxidized by means of atreatment with nitric acid or other suitable oxidizing agent to producethe orange colored silver ferricyanide image previously described.However, the unoxidized bleached image may be used, although thisprocedure is not on the whole as satisfactory. The image, preferably theoxidized or orange image, is next directly. converted into the desiredblueand-white image in a one step process, without the final treatmentin a hypo solution. The conversion mentioned takes place as a result ofimmersing the said orange image in a bath containing ions of a so-calledhypo such as sodium thiosulfate. and also ferrous ions preferably in aratio of about 2 to 1.5 respectively. The solution is acidified with theconventional acid mixtures used in the art for forming an acid hypos'olution. The presence of acid in the hypo solution serves the doublepurpose of preventing the precipitation of sulfur and preventinghydrolysis and its resultant precipitation of basic iron complexes.means of suitable mixtures of acids adapted to produce an acidified hyposolution, such as an acidified solution of sodium thiosulfate. Acidmixtures suitable for forming an acid hypo solution, are also suitablefor present purposes and are well known in the art. In particular, theacid hypo formula known as F-5a of the Eastman Kodak Company isrecommended. While variations within the scope of the foregoingexplanation are permissible, the ratio of hypo to ferrous ions should becontrolled.

It is believed that the blue color creating compound of the presentmodification differs from that of the form of the invention previouslydescribed. In the first place, since ferrous ions are used, I believethat relatively pure Tumbulls blue, that is, ferrous ferricyanide isproduced, The blue compounds believed to be intermixed with this productin the modification of the invention formerly described, namely,Prussian blue (ferriferrocyanidel and potassium ferriferrocyanideevidently are not present in this The former requires the presence offerric ions, while the comparatively strongly acidified solution of thepresent modification pre-, vents the formation of the latter which isformed in a basic or in a relatively weakly acidified solution. Sincebut one color creating compound is produced, the process embodying thepresent modification is easily controlled and produces images havingconstant color values and intensities.

A further advantage of this modification of i the invention resides inthe fact that the hypo (sodium thiosulfate. usually) dissolves thereaction products formed by the blue color forming reaction, other thanthe blue image forming compound and thus clears the blue image at theSuch precipitation may be prevented by i -Tumbulls blue (ferrousferricyanide).

compound. is unreacted and later lost by being dissolved oil.

SUMMARY OETHE PROCESS ANn A RECOMMENDED Annrrrormr. OPERATION fIHEREmBriefly speaking, two principal procedures embodying the presentinvention, have been pointed out. Both utilize a black-and-white imageand the bleaching thereof to form an image consisting of a complexsilver and iron cyanide. One procedure utilizes the oxidation of thiscomplex cyanide apparently to form silver ferricyanide and treats thisoxidized image in a single fixing and blue forming bath containing hypoand ferrous ions to form an image consisting chiefly of However, thefixing or clearing operation may be separately performed in a separatehypo bath after reacting the oxidized image with ferrous ions to form anunfixed blue image. The other procedure does not utilize the oxidizingstep and treats the unoxidized bleached image with ferrous or ferricions, preferably ferric ions, to form an image thought to consist of amixture of Prussian blue (ferroferricyanide) and Tumbulls blue(ferriferrocyanide) v However, apparently this image is composed chieflyof Prussian blue. In

this procedure also, hypo may be added to the blue forming bathcontaining ferrous or ferric ions, or the print may be fixed or clearedin a separate hypo bath.

If the oxidizing step is used, it is recommended that a silver halide beadded to the blue forming bath containing ferrous ions. This halide maybe added to the combined blue forming and fixing bath if it is used,that is, it may be added to a bath containing hypo in addition toferrous ions. Any silver halide may be employed but silver fiuoride doesnot operate satis-' its use is thereare insoluble. Therefore it isthought that the image produced is entirely Tumbulls blue(ferriferrocyanide) without mixtures of Prussian I .blue(ferroferricyanide) if the silver halide is added to the blue formingbath. The ionic concentration of the silver in the blue forming bath iskept depressed by this means until the silver complexes are removed bythe fixing agent, such as hypo, to leave a cleared blue image.

While the addition of a silver halide is one effective way of depressingthe'ionic concentration of silver in the blue forming bath. it willwater (about 18 C.).

readily be understood that other silver compounds having the same eifectmay be utilized. Such compounds are known to skilled chemists. Havingthus described my process or method in detail and having given thechemicalexplanation thereof insofar as such an explanation is known tome, the following examples of specific embodiments serve to revealtypical ways in which the invention may be practiced. These examples aregiven by way of illustration and not of limitation.

Each of the following examples presupposes the prior production of asilver black-and white" image and utilizes such a print as its startingpoint. It is to be understood, however,

that variations may be required in the examples given hereinto producethe desired blue colored image.

Example I Example I! (1) Follow steps 1 and 2 of Example 1.. (2) Placethe converted and washed print in an aqueous solution containingapproximately 17 to 35% by weight of nitric acid (100% HNOa).

' that if the initial image is of a different type r v thesolution,.washed. from about one to five min- (l) Immerse a photographicprint containing a developed silver image, which may or may not havebeen dried after development in an aqueous solution containingapproximately. by weight of potassium ferricyanide buffered with aboutgms. of sodium carbonate for each 250 cc. of solution. Allow the printto remain in this solution until the reaction has gone to completionwhich may be readily noted visibly inasmuch as the black image iscompletely converted into a grayish-white opaque one as viewed throughthe back of the print. These observations will reveal when all of thesilver has been converted. The, print may be removed, from the solutionand subjected to the following operations.

(2) Wash the converted print in cool water I (approximately 18 C.) forabout five minutes.

Gently running water is recommended, and a somewhat shorter washingperiod' has been found to produce no disadvantages.

(3) Place the converted and washed image in a bath prepared as follows:

Ferrous sulfate (C. P.) approx. gms '10 Sulfuric acid, dilute (about10%). approx. cc 10 Water approx. cc 100 Allow the print to remain inthis solution until its image is completely converted to an opaque blueone. However, the point at which complete conversion of the image fromwhite to blue has taken place may be readily noted by visual ob-vservation.

(4) The converted image is then rinsed in cool A rinse of one-half toone minute is recommended.

(5) The rinsed image is placed in a hypo bath,

the formula of which may be as follows:

Water at about 125 F cc 95.6 Sodium thiosulfate gms 36.25 Sodium sulflte"gms 2.25 Acetic acid (28%) cc 2 Boric acid (crystals) gms 1.1 Potassiumalum"-.. .gms 2.25 Cold water to make cc aLthough a 10% aqueous solution(10 gms. of sodium thiosulfate per 100 cc. of water) produces goodresults.

The now completed blue print is washed in running water at about 18 C.for approximately ten minutes and is dried. I

The print is allowed to remain in this solution until the reaction hasgone to completion. This change may be visibly noted inasmuch as thegrayish-white appearance of the print gradually assumes an orange color-The reaction hasv reached completion when the entire image has changedin color, which may be observed and normally takes place in about oneminute. The print may be removed from the solution for purposes of suchobservations. When the change has become completed the print is removedfrom utes in'cool water to remove soluble oxidation products and anexcess of the oxidizing agent and is then subjected to the followingoperations (3) Place the converted and washed image in a bath preparedas follows:

Ferrous ammonium sulfate approx. gms 16 Sulfuric acid, dilute (about10%) approx. cc 10 Water approx. liter 1 Allow the print to remain inthis solution until its image has been completely converted to an opaqueblueone.

(4) Follow steps 4, 5 and 6 .of Example 1. Example III (1) Follow steps1 and 2 of Example (2) Place the converted and washed image in a bathprepared as follows:

Silveriodide approx. gms 2 Sodium thiosulfate n, do; 2, Acid mixtureapprox. cc 20 Ferrous sulfate solution do 30 to make Water (atapproximately 18 C.)

- approx. cc 150 The ferrous sulfate solution referred to is made up asfollows: Sulfuric acid (concentrated). Approx. 1 cc. (1.8 gms.) Ferrousammonium sulfate Approx. 5 gms.

Water to make Approx. cc.

The acid mixture referred to is made up as follows: Water (about 52 C.)approx. liters Acetic acid (28% glacial acetic acid Y approx. cc- 940Sodium suliite v(desiccated) approx. gms 300 Boric acid (crystals) i doPotassium alum. -do 300 Cold water to make approx. liters 4 The acidmixture should be added before. the ferrous sulfate mixture in order toprevent undesired precipitation of sulfur or basic iron complexes.

' Allow the print to remain in this solution until its image iscompletely converted into a transparent blue-and-white one. The actualtime required for this reaction will vary. somewhat depending upon thetype of film employed. However, the point at which complete conversionhas taken place may be readily noted visibly. It is preferable to leavethe film in this solution for about two minutes after the image hasturned to a cleared, substantially transparent blue one. (3) The nowcompleted blue print is washed in running water at about 18 C. forapproximately two minutes and is allowed to dry.

IN Grimm.

In all of the above examples the following additional steps of procedureand technique have been found to be helpful although not indispensableto my process itself. The gelatin of the film may be hardened byconventional hardening baths either after development of theblack-and-white positive and before fixing thereof. after fixingthereof, after development only if no fixing step is utilized, or it maybe used after development and also after fixing. A bath of chrom alum orone of formaldehyde has been found to care for the extreme cases ofgelatin stripping and softening which otherwise might be adanger. inusing the present process with certain films. If a single hardening bathis considered preferable, the following one used after development ofthe black-and-white. positive may be employed. The solution is made asfollows:

Grams Water 200 Formalin (40%) 2 Sodium carbonate l The positive isimmersed in this bath for approximately five minutes and is then washedfrom three to five minutes before the various succeeding steps alreadydescribed are taken. This hardening step is particularly recommendedwhere the washing baths exceed the temperature control limits givenabove and in particular where such baths exceed 70 F., although it is tobe distinctly understood that such increased temperatures are to beavoided and may one embodiment of the invention the initial image isbleached and is prepared for the production of a blue color thereinduring a subsequent oxidation resulting from the treatment of the imagewith a suitable oxidizing agent. The

process also appears to be novel in that the oxidized image and printcontaining the same may be converted to a blue colored print and at thesame time cleared in a solution which contains a solvent forthecompounds which form no part of the blue color-producing compounds. Aspointed out, the blue color forming and clearing steps may be combinedwith beneficial results by ing 'an oxidizing agent adapted to form asilver ferricyanide image, and thereafter immersing the oxidizedbleached image in a solution containing sodium thiosulfate and a ferrousiron compound in a molecular ratio of approximately 2 to 1.5respectively.

2. A method of producing a blue photographicprint from an initialphotographic silver image comprising reacting said image underconditions suitable to produce an image composed chiefly of silverferrocyanide, oxidizing said reacted image by immersing the print in abath containing an oxidizing agent selected from the group consisting ofnitric acid, potassium permanganate, perchloric acid, and chromic acid,thereafter immersing said oxidized image in a bath containing a watersoluble iron compound and also a silver halide to form a substantiallywater insoluble blue colored compound of iron in chemical combinationwith an iron cyanide complex radical, said blue colored compound beingthereby directly substituted for the initial photographic, image.

3. A method of producing a blue photographic image from an initialdeveloped silver image.

- comprising reacting said image with a solution of prises bleachingsaid silver image to produce an insoluble silver complex iron cyanideimage, and

thereafter treating said bleached image with a solution containing athlosulfate and an iron compound to form ablue colored compound of ironand an iron cyanide complex radical, the

concentration of said thlosulfate to said iron using a bath whichcontains a hypo solution in which free iron ions are provided.

The blue color of the completed print has been separated and tentativelyidentified and has been found to consist chiefly of the compoundTurnbulls blue, Fea[Fe(CN) 612, which also probably contains arelatively small amount of Prussian blue, Fe4iFe(CN)la, and also arelatively small amount of potassium ferriferrocyanide, KFe'iFetCNh],which I believe to be due to some reduction of the iron in the ironcyanide complex probably by the action of organic reducers in the filmsuch for instance as the cellulosic carrier.

I claim:

l. The method of converting a developed photographic silver print to ablue color print, comprising bleaching the silver print to form a silverferrocyanide image followed by oxidizing the compound in said solutionbeing so controlled that the molecular ratio of the former to the latteris approximately 2 to 1.5.

5. A method of producing a blue photographic image from an initialsilver photographic image developed in a gelatin layer, which comprisesreacting said silver image under conditions suitable to produce ableachedimage composed substantially entirely of silver ferrocyanide,thereafter oxidizing said bleached image to produce an image composedsubstantially entirely of silver ferricyanide, and thereafter treatingthe resulting silver ferricyanide image with a solution containingferrous ions to form a blue image consisting substantially entirely offerroferricyanide.

6. A method of producing'a blue photographic 7. In a method of producinga blue photographic print from a, silver image developed in a gelatinlayer and which utilizes a precipitated compound of iron and a, complexiron cyanide as a color-producing means, the steps of first bleachingthe silver image to an image consisting substantially entirely of silverferrocyanide, and thereafter treating said bleached image with anoxidizing agent before the aforesaid color producing compound is formedto convert the bleached image to an image consisting substantiallyentirely of silver terricyanide.

8. A methodof producing a blue photographic image from-an initial silverphotographic image,

whichoomprises reacting said silver image under conditions suitable toproduce a bleached image composed substantially entirely of silverferrocyanide, thereafter treating said bleached image with a, solutioncontaining nitric acid to produce an image composed substantially entirely of silver ferricyanide, and thereafter reacting said silverferricyanide image with'a water soluble iron compound to produce a waterinsoluble blue image formed of iron in chemicalcombination with an ironcyanide complex radical.

ALBERT Ll BUN'I'ING.

